What thickness of choker/pea gravel is used as a filter between the underdrain and the soil media layer in bioretention?

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Multiple Choice

What thickness of choker/pea gravel is used as a filter between the underdrain and the soil media layer in bioretention?

Explanation:
In bioretention design, a choker layer of gravel sits directly on the underdrain to act as a filtration/transition between the underdrain and the soil media. This layer traps fine sediments that could clog the pipe while still allowing water to flow into the underdrain. Three inches provides enough thickness to filter out fines without unnecessarily reducing the pore space or storage in the soil media. If the layer is too thin, fines can migrate into the underdrain and cause clogging; if it’s thicker, you reduce storage and increase cost without much extra benefit. So, the standard thickness used is three inches.

In bioretention design, a choker layer of gravel sits directly on the underdrain to act as a filtration/transition between the underdrain and the soil media. This layer traps fine sediments that could clog the pipe while still allowing water to flow into the underdrain. Three inches provides enough thickness to filter out fines without unnecessarily reducing the pore space or storage in the soil media. If the layer is too thin, fines can migrate into the underdrain and cause clogging; if it’s thicker, you reduce storage and increase cost without much extra benefit. So, the standard thickness used is three inches.

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